Low-pressure area

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during December.

Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air becomes saturated as it rises, although the low-pressure area typically brings cloudy skies, which act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, which causes lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe (particularly in the British Isles and Netherlands), recurring low-pressure weather systems are typically known as "depressions".


Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere.[1] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areasanticyclogenesis.[2] Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term "cyclone" where circular pressure systems flow in the direction of the Earth's rotation,[3][4] which normally coincides with areas of low pressure.[5][6] The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size.[7][8] Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger-scale troughs, also called Rossby waves, are synoptic in scale.[9] Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature.[10] Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres.[11] All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure.[12]

Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak before occluding later in their life cycle as cold-core cyclones.[13][14][15][16] Polar lows are small-scale, short-lived atmospheric low-pressure systems that occur over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale weather-systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas- and oil-platforms. They are vigorous systems that have near-surface winds of at least 17 metres per second (38 mph).[17]

This depiction of the Hadley cell shows the process which sustains low-pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.

Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm-core with well-defined circulations.[18] Certain criteria need to be met for their formation. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft);[19] waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.[20] Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.[19] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.[19] Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.[19] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.[19] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[21] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[22]

In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in a low-pressure area called a thermal low.[23] Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean. This generates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it.[24] Similar rainfall is caused by the moist ocean-air being lifted upwards by mountains,[25] surface heating,[26] convergence at the surface,[27] divergence aloft, or from storm-produced outflows at the surface.[28] However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation. In winter, the land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling.[24] Monsoons resemble sea and land breezes, terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal.[29]


Mid-latitudes and subtropics

QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds on the poleward side of the occluded front.

Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic. The Arctic oscillation provides an index used to gauge the magnitude of this effect in the Northern Hemisphere.[30] Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans.[31] A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels there are an average of 37 cyclones in existence during any 6-hour period.[32] A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.[33] In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low-pressure weather systems are typically known as depressions.[34] These tend to bring wet weather throughout the year. Thermal lows also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert, the Mexican plateau, the Sahara, South America, and Southeast Asia.[23] The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.[31]

Monsoon trough

February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific

Elongated areas of low pressure form at the monsoon trough or intertropical convergence zone as part of the Hadley cell circulation.[35] Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest. It can reach as far as the 40th parallel in East Asia during August and 20th parallel in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure over the warmest part of the various continents.[36][37] The large-scale thermal lows over continents help create pressure gradients which drive monsoon circulations.[38] In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February,[39] oriented along a west-northwest/east-southeast axis. Many of the world's rainforests are associated with these climatological low-pressure systems.[40]

Tropical cyclone

Winston 2016-02-12 1200Z
Infrared image of a powerful southern hemisphere cyclone, Winston, near its initial peak intensity

Tropical cyclones generally need to form more than 555 km (345 mi) or poleward of the 5th parallel north and 5th parallel south, allowing the Coriolis effect to deflect winds blowing towards the low-pressure center and creating a circulation.[19] Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month while September is the most active month. November is the only month that activity in all the tropical cyclone basins is possible.[41] Nearly one-third of the world's tropical cyclones form within the western Pacific Ocean, making it the most active tropical cyclone basin on Earth.[42]

Associated weather

Coriolis effect10
Schematic representation of flow (represented in black) around a low-pressure area in the Northern hemisphere. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows

Wind is initially accelerated from areas of high pressure to areas of low pressure.[43] This is due to density (or temperature and moisture) differences between two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind.[44] Thus, stronger areas of low pressure are associated with stronger winds.

The Coriolis force caused by the Earth's rotation is what gives winds around low-pressure areas (such as in hurricanes, cyclones, and typhoons) their counter-clockwise (anticlockwise) circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure).[45] A cyclone differs from a hurricane or typhoon only on the basis of location.[46] A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean.[46] Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically, toward their centers.[44] A low-pressure area is commonly associated with inclement weather,[47] while a high-pressure area is associated with light winds and fair skies.[48] Tornados are often too small, and of too short duration, to be influenced by the Coriolis force, but may be so-influenced when arising from a low-pressure system.[49][50]

See also


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1970 North Indian Ocean cyclone season

The 1970 North Indian Ocean cyclone season had no bounds, but tropical cyclones in the North Indian Ocean tend to form between April and December, with peaks in May and November. The 1970 season saw a total of seven cyclonic storms, of which three developed into severe cyclonic storms. The Bay of Bengal was more active than the Arabian Sea during 1970, with all of the three severe cyclonic storms in the season forming there. Unusually, none of the storms in the Arabian Sea made landfall this year. The most significant storm of the season was the Bhola cyclone, which formed in the Bay of Bengal and hit Bangladesh on November 12. The storm killed at least 500,000, making it the deadliest tropical cyclone in recorded history.

2000 Pacific typhoon season

The 2000 Pacific typhoon season marked the first year using names contributed by the World Meteorological Organization. It was a rather below-average season, producing a total of 23 tropical storms, 13 typhoons and 4 intense typhoons. The season ran throughout 2000, though typically most tropical cyclones develop between May and October. The season's first named storm, Damrey, developed on May 7, while the season's last named storm, Soulik, dissipated on January 4 of the next year.

The scope of this article is limited to the Pacific Ocean to the north of the equator between 100°E and the 180th meridian. Within the northwestern Pacific Ocean, there are two separate agencies that assign names to tropical cyclones, which often results in a storm having two names. The Japan Meteorological Agency (JMA) will name a tropical cyclone should it be judged to have 10-minute sustained wind speeds of at least 65 km/h (40 mph) anywhere in the basin, whilst the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) assigns names to tropical cyclones which move into or form as tropical depressions in their area of responsibility, located between 115°E and 135°E and between 5°N and 25°N, regardless of whether or not the tropical cyclone has already been given a name by the JMA. Tropical depressions monitored by the United States' Joint Typhoon Warning Center (JTWC) are given a number with a "W" suffix.

2005 Azores subtropical storm

The 2005 Azores subtropical storm was the 19th nameable storm and only subtropical storm of the record-breaking 2005 Atlantic hurricane season. It was not officially named by the US National Hurricane Center as it was operationally classified as a non-tropical low. The storm developed in the eastern Atlantic Ocean out of a low-pressure area that gained subtropical characteristics on October 4. The storm was short-lived, crossing over the Azores later on October 4 before becoming extratropical again on October 5. Neither damages nor fatalities were reported during that time. After being absorbed into a cold front, the system went on to become Hurricane Vince, which affected the Iberian Peninsula.

Months after the hurricane season, when the National Hurricane Center was performing its annual review of the season and its named storms, forecasters Jack Beven and Eric Blake identified this previously unnoticed subtropical storm. Despite its unusual location and wide wind field, the system had a well-defined centre convecting around a warm core—the hallmark of a subtropical storm.

2014 North Indian Ocean cyclone season

The 2014 North Indian Ocean cyclone season was an event in the annual cycle of tropical cyclone formation. The season included two Very Severe Cyclonic Storms, both in October, and one other named cyclonic storm. Cyclone Hudhud is estimated to have caused US$3.58 billion in damage across eastern India, and more than 120 deaths.

The scope of this article is limited to the Indian Ocean in the Northern Hemisphere, east of the Horn of Africa and west of the Malay Peninsula. There are two main seas in the North Indian Ocean — the Arabian Sea to the west of the Indian subcontinent, abbreviated ARB by the India Meteorological Department (IMD); and the Bay of Bengal to the east, abbreviated BOB by the IMD. The official Regional Specialized Meteorological Centre in this basin is the India Meteorological Department (IMD), while the Joint Typhoon Warning Center releases unofficial advisories. On average, four to six storms form in this basin every season.

2017 North Indian Ocean cyclone season

The 2017 North Indian Ocean cyclone season was a below average season in the annual cycle of tropical cyclone formation. This season produced only three named storms, of which one only intensified into a very severe cyclonic storm. The North Indian Ocean cyclone season has no official bounds, but cyclones tend to form between April and December, with the two peaks in May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northern Indian Ocean. The season began with the formation Cyclone Maarutha on April 15, and ended with the dissipation of a deep depression on December 9.

The scope of this article is limited to the Indian Ocean in the Northern Hemisphere, east of the Horn of Africa and west of the Malay Peninsula. There are two main seas in the North Indian Ocean – the Arabian Sea to the west of the Indian subcontinent, abbreviated ARB by the India Meteorological Department (IMD); and the Bay of Bengal to the east, abbreviated BOB by the IMD. The official Regional Specialized Meteorological Centre in this basin is the IMD, while the Joint Typhoon Warning Center (JTWC) releases unofficial advisories. On average, three to four cyclonic storms form in this basin every season.

Alberta clipper

An Alberta-clipper (also known as a Canadian Clipper) is a fast moving low pressure area weather system which generally affects the central provinces of Canada, as well as parts of the Upper Midwest, Great Lakes, and Northeastern United States regions, precipitating a sudden temperature drop and sharp winds. Alberta clippers take their name from Alberta, the province from which they appear to descend, and from clipper ships of the 19th century, one of the fastest ships of that time.

Braer Storm of January 1993

The Braer Storm of January 1993 was the most intense extratropical cyclone ever recorded over the northern Atlantic Ocean. Developing as a weak frontal wave on 8 January 1993, the system moved rapidly northeast. The combination of the absorption of a second low-pressure area to its southeast, a stronger than normal sea surface temperature differential along its path, and the presence of a strong jet stream aloft led to a rapid strengthening of the storm, with its central pressure falling to an estimated 914 hPa (914 mb; 27.0 inHg) on 10 January. Its strength was well predicted by forecasters in the United Kingdom, and warnings were issued before the low initially developed.

Gale-force winds covered the far northern Atlantic between Western Europe and Atlantic Canada, due to the intensity of this storm, with hurricane-force winds confined near its center of circulation. After reaching its peak intensity, the system weakened as it moved into the far northeast Atlantic, dissipating by 17 January. This storm caused severe blizzards across much of Scotland. It also led to the final breakup of the oil tanker MV Braer, from which the storm derived its name; it had been stranded in rocks off the Shetland Islands by a previous storm nearly a week beforehand.

Colorado low

A Colorado low is a low-pressure area that forms in southeastern Colorado or northeastern New Mexico, typically in the winter. After forming, the system moves across the Great Plains. Colorado lows produce heavy wintry precipitation, and have a general east to northeast movement, impacting regions as far north as Winnipeg and as far east as the Atlantic coast. If upper level conditions are right, the jet stream can push the low farther south, bringing wintry precipitation as far as Texas. When pushed this far south, the system is often referred to as a "blue norther". On the more typical track, a Colorado low can be similar to an Alberta clipper. Winter Colorado lows are responsible for a majority of the snow that the Midwest receives; however, summer systems can trigger vast, long-lasting thunderstorms. Spring and early summer Colorado low cyclogenesis can precipitate significant "syoptically evident" tornado outbreaks over the Great Plains and Midwest.

Cyclone Herwart

Cyclone Herwart was a European windstorm that affected Southern Denmark, Germany, Poland, Austria, Hungary and the Czech Republic on 28–29 October 2017. Named by the Free University of Berlin Meteorology Department, the storm was an extratropical cyclone formed as a secondary low to a more northerly centre of low pressure named Grischa coming southward from the Svalbard Islands region, the latter splitting in two low-pressure areas late on 28 October. The center of Herwart started rotating counterclockwise around the main low pressure area, passing over Norway, Sweden, Latvia and then losing power while moving over western Russia.In Denmark, which was hit on 28 October, the storm was named Ingolf. In Hungary, the storm was named Nárcisz (Narcissus), a Hungarian female name whose name day is on 29 October.

Gulf low

A Gulf low or Texas Low is a low pressure area that forms or intensifies over the Gulf of Mexico.

Because they move northward from over or near the Gulf of Mexico, these storm systems are capable of transporting copious amounts of moisture with them. At their strongest, these storm systems are even more potent snowfall producers than panhandle hooks, primarily because of the mixing of Atlantic Ocean moisture into the storm system once they cross the Appalachian Mountains. Because of the general west to east movement of weather systems in the mid latitudes of the northern hemisphere, Gulf Lows rarely affect areas west of the Mississippi River. One such exception was the Halloween Blizzard of 1991.

There are two types of "Gulf Lows": West and East Gulf Cyclones or Types Ga and Gb. As western storms (Alberta, North and South Pacific and Rocky Mountain) move eastward, a trailing frontal remnant is often left in the Gulf of Mexico or the adjacent coastal states. When the frontal trough from a new cyclone enters this area, cyclogenesis often occurs on the Gulf front. This newly formed Gulf wave that moves northeastward and dominates the weather over the eastern United States. One of the basic differences between the two types is that the Ga track is west of the Appalachians

and the Gb track is east of the Appalachians.

The Ga type is characterized by an upper level trough in the central and eastern part of the United States which steers the newly formed wave north and northeast over the western side of the Appalachians accompanied by widespread precipitation to the north. This type is most frequently observed in the winter and early spring.

The 1993 North American Storm Complex started out as a Gulf Low (East Gulf Low or Gb) before it eventually evolved into a nor'easter. It initially formed in the western Gulf of Mexico and moved towards Florida before turning to the north and becoming a nor'easter. Another large gulf low storm was the Great Blizzard of 1978, which brought one of the lowest non-tropical barometric pressures recorded in the United States.

High-pressure area

A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment.

Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center.

However, because the planet is rotating underneath the atmosphere, and frictional forces arise as the planetary surface drags some atmosphere with it, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect, or the merely apparent force that arise when the observer is in a rotating frame of reference. Viewed from above this twist in wind direction is in the same direction as the rotation of the planet.

The strongest high-pressure areas are associated with cold air masses which push away out of polar regions during the winter when there is less sun to warm neighboring regions. These Highs change character and weaken once they move further over relatively warmer water bodies.

Somewhat weaker but more common are high-pressure areas caused by atmospheric subsidence, that is, areas where large masses of cooler, drier air descend from an elevation of 8 to 15 km after the lower temperatures have precipitated out the water vapor.

Many of the features of Highs may be understood in context of middle- or meso-scale and relatively enduring dynamics of a planet's atmospheric circulation. For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells. Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most of the world's deserts.

On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols.

Hurricane Daniel (2006)

Hurricane Daniel was the second strongest hurricane of the 2006 Pacific hurricane season. The fourth named storm of the season, Daniel originated on July 16 from a tropical wave off the coast of Mexico. It tracked westward, intensifying steadily to reach peak winds of 150 mph (240 km/h) on July 22. At the time, the characteristics of the cyclone resembled those of an annular hurricane. Daniel gradually weakened as it entered an area of cooler water temperatures and increased wind shear, and after crossing into the Central Pacific Ocean, it quickly degenerated into a remnant low-pressure area on July 26, before dissipating two days later.

Initial predictions suggested that the cyclone would pass through the Hawaiian Islands as a tropical storm; however, Daniel's remnants dissipated southeast of Hawaii. The storm brought light to moderate precipitation to the Island of Hawaii and Maui, causing minor flooding, although no major damage or fatalities were reported.

Line echo wave pattern

A line echo wave pattern (LEWP) is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south (or equatorward) of a mesoscale low-pressure area with a rotating "head". LEWP often are associated with a multiple-bow serial derecho and often produce tornadoes, some of which can be strong. The existence of a LEWP on radar means that a serial derecho has developed or is likely to develop soon, much as a hook echo indicates the same for a tornado.

Thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, the Sahara, over north-west Argentina in South America, over the Kimberley region of north-west Australia, the Iberian peninsula, and the Tibetan plateau.

Over land, intense, rapid solar heating of the land surface results in heating of the lowest layers of the atmosphere via reradiated energy in the infrared spectrum. The resulting hotter air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in the formation of a low pressure area. Elevated areas can enhance the strength of the thermal low as they warm more quickly than the atmosphere which surrounds them at the same altitude. Over the water, instability lows form during the winter when the air overlying the land is colder than the warmer water body. Thermal lows tend to have weak circulations, and can extend to 3,100 metres (10,200 ft) in height. Thermal lows over the western and southern portions of North America, northern Africa, and southeast Asia are strong enough to lead to summer monsoon conditions. Development of thermal lows inland of the coastline lead to the development of sea breezes. Sea breezes combined with rugged topography near the coast can encourage poor air quality.

Timeline of the 2005 Pacific hurricane season

The 2005 Pacific hurricane season was the least active season since the 2001 season, producing 16 tropical depressions; 15 of which became tropical storms or hurricanes. The season officially started on May 15, 2005 in the eastern Pacific, designated as the area east of 140°W, and on June 1, 2005 in the central Pacific, which is between the International Date Line and 140°W, and lasted until November 30, 2005. These dates typically limit the period of each year when most tropical cyclones form in the eastern Pacific basin. This timeline documents all the storm formations, strengthening, weakening, landfalls, extratropical transitions, as well as dissipation. The timeline also includes information which was not operationally released, meaning that information from post-storm reviews by the National Hurricane Center, such as information on a storm that was not operationally warned on, has been included.

The first storm of the season, Hurricane Adrian, formed off the southwest coast of El Salvador and made the closest approach of any hurricane to the country on record. Most of June was quiet until the end of the month, when two tropical storms developed. July remained inactive as well, as only two tropical storms formed. In August, the activity picked up somewhat; the Central Pacific had its first and only depression of the year and four tropical storms, two of which became hurricanes, forming in the Eastern Pacific. September was the most active month of the year; six tropical storms formed, four of which became hurricanes, and two of the hurricanes strengthened further to become the only major hurricanes of the year. The strongest storm of the season was Hurricane Kenneth, whose remnants briefly threatened Hawaii near the end of the month. Activity sharply dropped off in October as only one tropical depression formed. No storms formed in November in both basins, and the season ended on November 30.

Tropical Depression One (2009)

Tropical Depression One was the first tropical cyclone to develop during the 2009 Atlantic hurricane season. Upon being declared a tropical depression on May 28, it marked the third time that a pre-season storm formed in three consecutive years. Originating from a disorganized area of low pressure off the coast of North Carolina, Tropical Depression One quickly developed over the Gulf stream. After attaining winds of 35 mph (55 km/h) along with a minimum pressure of 1006 mbar (hPa; 29.71 inHg), the depression began to weaken due to increasing wind shear and cooling sea surface temperatures. During the afternoon of May 29, convection associated with the system was significantly displaced from the center of circulation; this led the National Hurricane Center to issue their final advisory on the depression as it had degenerated into a remnant-low pressure area. As a tropical cyclone, Tropical Depression One had no effects on land; however, the precursor to the depression brought minor rainfall and light winds to parts of coastal North Carolina. Its track, formation, and timing were relatively similar to Tropical Storm One of the 1940 Atlantic hurricane season.

Tropical Depression Wilma (2013)

Tropical Depression Wilma, also referred to as 30W and Depression BOB 05, was a weak but long-lived tropical cyclone that traveled from the Northwest Pacific Ocean to the North Indian Ocean in 2013. Forming east of Palau on November 1, the tropical depression passed through the Philippines on November 4 and emerged into the South China Sea on the next day. Without intensification, the system made landfall over Vietnam on November 6 and arrived at the Gulf of Thailand on November 7.

On November 8, the tropical depression crossed the Malay Peninsula and emerged into the Bay of Bengal. Being a low-pressure area later, it was struggling to develop until intensifying into a depression on November 13. The system made landfall over India on November 16 and caused 16 fatalities, before it weakened into a low-pressure area, and then entered the Arabian Sea on the next day.

Tropical Storm Kiko (2007)

Tropical Storm Kiko was a strong tropical storm that capsized a boat off the western coast of Mexico, killing at least 15 people. The 15th and final tropical cyclone of the 2007 Pacific hurricane season, Kiko developed out of a tropical wave that formed off the coast of Africa on September 26 and traversed the Atlantic. The wave crossed over Central America and entered the Pacific Ocean on October 8, where it spawned Tropical Depression 15-E on October 15. The depression drifted to the south over the next day before briefly being declared Tropical Storm Kiko. It subsequently weakened into a tropical depression, but later reattained tropical storm intensity. By October 18, Kiko was forecast to make landfall along the western Mexican coastline as a moderate tropical storm. However, the cyclone turned to the west and reached its peak intensity of 70 mph (110 km/h) on October 20. The tropical storm slowly weakened to a remnant low-pressure area by October 24 and completely dissipated on October 27 without making landfall.

Wake low

A wake low, or wake depression, is a mesoscale low-pressure area which trails the mesoscale high following a squall line. Due to the subsiding warm air associated with the systems formation, clearing skies are associated with the wake low. Once difficult to detect in surface weather observations due to their broad spacing, the formation of mesoscale weather station networks, or mesonets, has increased their detection. Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough. When the squall line is in the process of decay, heat bursts can be generated near the wake low. Once new thunderstorm activity along the squall line concludes, the wake low associated with it weakens in tandem.

Northern Hemisphere
Southern Hemisphere

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